The NTSC (National Television System Committee) system which is one of conventional television broadcasting systems, the aspect ratio of a video signal to be 4:3. On the other hand, high-definition (high-vision) television systems, such as the MUSE (Multiple Sub-Nyquist Sampling Encoding) system and the EDTV (Extended Definition Television) system are improved over the NTSC system and the like.
In high-definition television systems, the aspect ratio of a video signal is defined at 16:9. For this reason, even in a field of television sets for displaying a video signal, apparatuses called wide TVs (Television) capable of displaying a wide screen are rapidly spreading recently.
Many software packages, such as video tapes and laser disks, which record video signals with aspect ratios wider than 4:3, such as movie sizes, are commercially available. As shown in FIG. 1, video software of this type often has a display form called a letter-box screen which contains black non-image portions P1 and P2 without any video above and below a main screen S in display on a screen with an aspect ratio of 4:3.
The video software having a wide screen are expected to increase in the future. In this case, video signals having various aspect ratios of 4:3, 16:9, or a movie size, and this poses a problem. In fact, there are letter-box screens of various types, i.e., videos having various aspect ratios and various display forms.
For example, FIGS. 2A and 2B show two types of movie software having an aspect ratio of 1.85:1, i.e., a so-called vista size but different subtitle positions. FIGS. 2C and 2D show two types of movie software having an aspect ratio of 2.35:1, i.e., a so-called cinemascope size but different subtitle positions. In addition, the display form variously changes in accordance with the number of subtitle lines, or domestic or foreign movie software. Dubbed software need no subtitle, and its display form changes accordingly. For example, the number of lines of an upper-side non-image portion varies from about 10 to 50.
In a conventional wide TV, various display means have been examined to cope with the above-mentioned various wide video software. For example, FIG. 3A shows a mode (normal mode) in which non-image portions are formed on the left and right sides of a video. FIG. 3B shows a mode (zoom mode) in which a video image is extended in the vertical and horizontal directions to display the horizontal video region on the display screen to its horizontal limit while cutting the upper and lower portions of the video. FIG. 3C shows a mode (live mode) in which the horizontal video region of a video is displayed on the display screen to its horizontal limit while the vertical video region is extended at a certain ratio. FIG. 3D shows a mode (full mode) in which a video is extended only in the horizontal direction, thereby displaying the video region on the display screen to its horizontal limit.
When a letter-box screen video signal is displayed on a wide TV, the video signal is displayed in any one of the above four modes. In the normal mode, however, a region where the video is actually displayed is small, so advantages in large screen display decrease. In the zoom mode, the upper and lower portions of the video with an aspect ratio of 4:3 is cut. However, become these cut portions have no picture pattern from the first, no problem arises. In the zoom mode, the picture pattern portion is conveniently displayed on the screen to its limit.
However, movie software with a video signal containing a subtitle at the lower portion of the screen or a picture pattern portion shifted to the upper or lower side is displayed, important picture pattern information may be missed in the zoom mode. In this case, processing for changing the extension ratio or vertically moving the displayed picture pattern must be performed. In any case, because the cutting ratio of the upper and lower portions of the video is large in the zoom mode, adjustment is required.
In the live mode, the upper and lower portions of a video with an aspect ratio of 4:3 are slightly cut, and the circularity is slightly extended in the horizontal direction. However, become the upper and lower cut portions of the video are smaller than those in the zoom mode, adjustment such as picture pattern movement need not be performed. Additionally, in the live mode, the picture pattern is slightly elongated in the horizontal direction, though no serious problem is posed. In the full mode, a video with an aspect ratio of 4:3 is not cut at all. However, as compared to the live mode, the picture pattern is elongated in the horizontal direction. For this reason, employing is not desirable.
In any of the above-described display modes, the viewer must switch the mode while viewing the picture pattern displayed on the screen. More specifically, the viewer must select an optimum display mode every time the aspect ratio of an input video signal changes.
Traditionally, the horizontal and vertical amplitude levels of a TV set are automatically controlled in accordance with the aspect ratio of video software. For example, Japanese Patent Application No. 63-193779 describes a technique for detecting a letter-box screen, in which a clock counter detects a blanking (non-image portion) width when the luminance signal level is lower than a predetermined DC voltage level. If a vertical sync signal is detected during the blanking period, a non-image portion is determined.
As described above, video software that has a letter-box screen also contains blanking (non-image portion) periods above and below the screen. However, the luminance signal level of this non-image portion period is not clearly defined, and various levels are detected. In an actual examination of video software with various letter-box screens, the luminance signal levels during the non-image portion period often increase to almost 15 (IRE). Because the black level in the NTSC system is 5 (IRE) as broadcasting standard, these signal levels are higher than the black level.
On the other hand, even in video software with an aspect ratio of 4:3, the luminance signal level in a dark screen decreases to the black level, as a matter of course. Particularly, in a dark normal screen with an aspect ratio of 4:3 in dark red or blue, many video signals at levels lower than the signal level of a non-image portion are present. For this reason, erroneous determinations tend to occur when determining a letter box screen only on the basis of the luminance signal level of an upper/lower non-image portion.
As shown in FIG. 4A, a video with an aspect ratio of 4:3, is erroneously determined as a letter-box video. The vertical amplitude level is increased, and the upper and lower ends of the picture pattern are omitted. When a letter-box video is erroneously determined as a video with an aspect ratio of 4:3, the display screen of a wide TV screen cannot be effectively used.
FIG. 5A depicts a letter-box signal. A vertical sync period, a vertical blanking period, and non-image periods are represented by z, y, s, and t, respectively. Generally, when the time constant is insufficient in an AC (alternate current) coupling circuit, a sag occurs, as shown in FIG. 5B. More specifically, in the letter-box signal, the DC voltage level monotonously increases at the upper and lower non-image portions s and t and during the vertical blanking period y while the DC voltage level monotonously decreases at the image portion. When the waveform of this letter-box signal is simply sliced with a DC voltage, a slice level indicated by a broken line in FIG. 5B crosses the non-image portion periods s and t, the presence/absence of the non-image portion cannot be properly detected.
However, such a sag sometimes occurs in a signal source of a VTR (Video Tape Recorder) or a laser disk player and cannot always be coped with a TV set. For this reason, an apparatus which properly determines the non-image portion periods s and t even in a signal waveform with a sag and controls the vertical amplitude level or the horizontal amplitude level is desired.
When a letter-box screen video signal is poor in quality, it is difficult to determine the letter-box screen. FIG. 6A shows an example in which the clamp performance is degraded midway along a video signal transmission line, resulting in the variations in DC component. For a video signal in this case, the edge portion of the upper/lower non-image portion is largely lifted. For this reason, the lifted portion cannot be determined as a vertical blanking period and is erroneously determined as a screen with an aspect ratio of 4:3. FIG. 6B shows an example in which noise of an AC component is included in a video signal. In this case, the upper/lower portion becomes bright or dark and is erroneously determined as a screen with an aspect ratio of 4:3. FIG. 6C shows an example in which noise of a control signal multiplexed in video software, or noise in head switching of a VTR is included. In this case as well, a discontinuity is determined on the basis of the continuity of the vertical blanking level, and the signal is erroneously determined as a screen with an aspect ratio of 4:3.
As described above, the conventional detection means for detecting a letter-box screen only references the luminance signal level in determining whether non-image portions are present above and below the screen. Therefore, erroneous determination tends to occur. It is strongly desirable to properly determine the presence/absence of a non-image portion even in a video signal with a sag. In addition, even when the vertical blanking level of a video signal varies, or a discontinuity occurs due to noise, it is necessary to properly determine a letter-box screen.
The present invention has been made in consideration of the above situations. Therefore, the objective of the present invention is to provide an excellent letter-box screen detection apparatus which can properly determine the presence/absence of a non-image portion and accurately perform determination of a letter-box screen.